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EDEXCEL IGCSE / CERTIFICATE IN PHYSICS 2-4

Electrical ResistanceEdexcel IGCSE Physics pages 82 to 88

June 17th 2012

 All content applies for Triple & Double Science

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Edexcel Specification

Section 2: Electricity

c) Energy and potential difference in circuits

describe how current varies with voltage in wires, resistors, metalfilament lamps and diodes, and how this can be investigatedexperimentally

describe the qualitative effect of changing resistance on the current ina circuit

describe the qualitative variation of resistance of LDRs withillumination and of thermistors with temperature

know that lamps and LEDs can be used to indicate the presence of acurrent in a circuit

know and use the relationship:voltage = current × resistance V = I × R

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Resistance

Resistance is the oppositionthat an electrical device has

to the flow of electrical

current.

 All devices have some

resistance. A resistor is a

device that has a particularresistance.

a resistor

circuit symbol for

a resistor

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Resistance equation

resistance = voltage

current

R =  V

I

Where:

voltage is in volts (V)

current is in amperes (A)resistance is in ohms (Ω)

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  Also:

V = I x R and:

I = V

R V

RI

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Measuring Resistance

The resistance of acomponent can be

found by measuring

the current through,and voltage across,

the component.Circuit used for measuring the

resistance of an indicator lamp

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Question 1

Calculate the resistance of a lamp if avoltage of 12V causes a current of 3A toflow through the lamp.

resistance = voltage

current

= 12V / 3Aresistance = 4 ohms  (4Ω)

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Question 2

Calculate the resistance of a heater if a voltage of230V causes a current of 200mA to flow throughthe heater.

resistance = voltagecurrent

= 230V / 200mA

= 230V / 0.200Aresistance = 1150 Ω 

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Question 3

Calculate the voltage across a resistance of40 Ω when a current of 5A is flowing.

V = I x R

= 5A x 40Ω 

voltage = 200V 

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Question 4

Calculate the current flowing through a wireof resistance of 8 Ω when a voltage of 12V isconnected to the wire.

current = voltage

resistance= 12V / 8Ω 

current = 1.5A 

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 Answers

V I R

20 V 4 A

5 A 40 Ω 

300 V 0.20  50 Ω 

8 V 500 mA

3 kV 150 Ω 

4 mA 30 kΩ 

5 Ω 

200 V

6 A

16 Ω 

20 A120 V

Complete:

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Current-voltage graphs

These are used to show how the

current through a componentvaries with the applied voltage.

The circuit opposite could be used

to obtain a current-voltage graph

of a resistor.

The variable resistor is used to

apply a range of voltages across

the resistor.

Typical results:

Voltage (V) 0.00 0.30 0.60 0.90 1.20 1.50

Current (A) 0.00 0.04 0.08 0.12 0.16 0.20

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The current-voltage graph of a wire or a

fixed resistor at a constant temperature

The graph is a straightline through the origin.

The wire or resistor

obeys Ohm’s law whichstates that that thecurrent is proportional tothe voltage.

I

V

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Choose appropriate words to fill in the gaps below:

An electric ________ will only flow around a circuit if there

are no ______ in the circuit.

All components have __________. The greater the resistance

the ________ is the current for the same applied voltage.

Resistance is measured in ______.

A current – 

 voltage graph for a ________ is a straight line

through the _______. This shows that the current through

the resistor is ___________ to the applied voltage.

resistance currentresistor ohmsgaps originsmaller

WORD SELECTION:

proportional

resistance

current

resistor

ohms

gaps

origin

smaller

proportional

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Diode

Current only flows through a diodein one direction. (Shown by thearrow on its symbol)

The diode has a very high

resistance in the reverse direction.

 A light emitting diode (LED) emitslight when electric current flows.

I

V

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Thermistor

The resistance of athermistor decreases asthe temperatureincreases.

The higher temperatureline therefore has agreater slope than thelower temperature case.

I

V

HOT

COLD

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Choose appropriate words to fill in the gaps below:

The resistance of a filament lamp _________ when the lamp

comes on and the filament rises in ___________.A ______ only allows electric current to flow one way. The

allowed direction is shown by the _______ on its circuit

symbol.

The ________ of a thermistor decreases if its temperature is

increased. The resistance of a LDR _________ if the

 _______level is increased.

resistancedecreasesarrow diode lightincreases

WORD SELECTION:

temperature

resistance

decreases

arrow

diode

light

increases

temperature

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Online SimulationsSimple demonstration of the effect of a variableresistor  - Freezeway.com

Battery-Resistor Circuit - PhET - Look inside aresistor to see how it works. Increase thebattery voltage to make more electrons flowthough the resistor. Increase the resistance toblock the flow of electrons. Watch the currentand resistor temperature change

Light bulb being controlled by a variableresistor

Electric circuits with resistors - series & parallel

with meters - netfirmsVariable resistor with an ammeter & avoltmeter

Resistance measurement demo - MolecularExpressions

Ohm's Law - PhET - See how the equationform of Ohm's law relates to a simple circuit. Adjust the voltage and resistance, and see thecurrent change according to Ohm's law. The

sizes of the symbols in the equation change tomatch the circuit diagram.

Ohm's Law - Fendt

Simple V-I circuit with a light bulb -Freezeway.com

Resistance in a Wire - PhET - Learn about thephysics of resistance in a wire. Change itsresistivity, length, and area to see how theyaffect the wire's resistance. The sizes of thesymbols in the equation change along with thediagram of a wire.

Resistance Wire Simulation - by KT - Designedfor the GCSE Investigation but can also beused to show the affect of source resistanceand to show power supply maximum power.Introduction . Worksheets for GCSEinvestigation

Conductivity - PhET - Experiment withconductivity in metals, plastics andphotoconductors. See why metals conduct andplastics don't, and why some materials conductonly when you shine a flashlight on them.

BBC AQA GCSE Bitesize Revision:

Calculating resistance Changing resistance 

Filament lamp with IV curve 

Thermistors & LDRs 

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Electrical ResistanceNotes questions from pages 82 to 88

1. Give the equation for resistance and state the units used. (see

page 83)2. Describe how the resistance of a thermistor and LDR can be

varies. (see page 84)

3. With the aid of a circuit diagram describe an experiment to find outhow the current through a wire varies with the voltage across it.(see pages 85 and 86)

4. Sketch graphs showing how current varies with voltage with a (a)fixed resistor; (b) lamp; (c) diode. (see pages 86 and 87)

5. What is Ohm’s law? (b) How does a graph show whether or notOhm’s law is being obeyed? (see page 85) 

6. Answer the questions on page 88.

7. Verify that you can do all of the items listed in the end of chapterchecklist on page 88.